The prior art has recognized the need for thermostatically controlled hydraulic drain valves, particularly, for draining water systems in buildings and equipment to prevent breakage due to freezing. Typically, these drain valves are powered by a springtrip mechanism or by an expandable bellows mechanism, either of which senses a change in temperature and moves a suitably arranged valve piston from a first position which connects the system to the supply, to a second position which connects the system to a drain. Another type of valve considered by the prior art requires a thermostat and a source of external electrical power.